A magnetic disk drive includes circular data tracks on a rotating magnetic disk and read and write heads that may form a merged head attached to a slider on a positioning arm. During a read or write operation, the merged head is suspended over the magnetic disk on an air bearing surface (ABS). The sensor in a read head is a critical component since it is used to detect magnetic field signals by a resistance change. The resistance change is produced by a giant magnetoresistance (GMR) effect which is based on a configuration in which two ferromagnetic layers are separated by a non-magnetic conductive layer in the sensor stack. One of the ferromagnetic layers is a pinned layer in which the magnetization direction is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) or pinning layer. The second ferromagnetic layer is a free layer in which the magnetization vector can rotate in response to external magnetic fields. The rotation of magnetization in the free layer relative to the fixed layer magnetization generates a resistance change that is detected as a voltage change when a sense current is passed through the structure. In a CPP configuration, a sense current is passed through the sensor in a direction perpendicular to the layers in the stack. Alternatively, there is a current-in-plane (CIP) configuration where the sense current passes through the sensor in a direction parallel to the planes of the layers in the sensor stack.
Ultra-high density (over 100 Gb/in2) recording requires a highly sensitive read head. To meet this requirement, the CPP configuration is a stronger candidate than the CIP configuration which has been used in recent hard disk drives (HDDs). The CPP configuration is more desirable for ultra-high density applications because a stronger output signal is achieved as the sensor size decreases, and the magnetoresistive (MR) ratio is higher than for a CIP configuration.
In the CPP GMR head structure, a bottom spin valve film stack is generally employed for biasing reasons as opposed to a top spin valve where the free layer is below a copper spacer and the pinned layer is above the copper spacer. Additionally, a CoFe/NiFe composite free layer is conventionally used following the tradition of CIP GMR improvements. An important characteristic of a GMR head is the MR ratio which is dR/R where dR is the change in resistance of the spin valve sensor and R is the resistance of the spin valve sensor before the change. A higher MR ratio is desired for improved sensitivity in the device and this result is achieved when electrons in the sense current spend more time within the magnetically active layers of the sensor. Interfacial scattering which is the specular reflection of electrons at the interfaces between layers in the sensor stack can improve the MR ratio and increase sensitivity. Unfortunately, the MR ratio is often very low (<5%) in many CPP-GMR spin valve structures. An MR ratio of about 10% is required for advanced applications.
A synthetic anti-parallel (SyAP) pinned layer configuration represented by AP2/coupling/AP1 is preferred over a single pinned layer because a smaller net magnetic moment is possible for a SyAP layer and that means greater exchange coupling between the AFM layer and adjacent AP2 layer. It is also known in the art that a laminated AP1 layer made of alternating CoFe and thin Cu layers can improve the MR ratio in CPP-GMR heads. The resulting CPP-GMR bottom spin valve is represented by a seed/AFM/pinned/spacer/free/cap configuration where seed is a seed layer, the spacer is a copper layer, the free layer is a CoFe/NiFe composite, and the pinned layer has an [AP2/coupling/AP1] SyAP configuration in which Ru is the coupling layer, the AP2 layer is made of CoFe, and the AP1 layer is a [CoFe/Cu]nCoFe laminated layer.
Another important consideration for CPP-GMR read heads is electromigration (EM) performance. For CPP spin valve structures having an AP2/coupling/AP1 pinned layer configuration, Fe rich CoFe alloys such as Fe50Co50 or Fe70Co30 in the AP1 layer are known to be effective in enhancing the MR ratio. However, Fe rich CoFe alloys usually result in poor EM performance. Therefore, it is very desirable to improve the EM performance of a CPP-GMR head having a Fe rich AP1 layer.
U.S. Pat. No. 5,715,121 discloses a further means of CPP-GMR improvement by inserting a confining current path (CCP) layer in the copper spacer by segregating metal path and oxide formation. The distribution of a Cu conductor in an electrical insulator (oxide) may vary as long as electrical conduction in the direction of film normal is larger than that in the direction of the film plane.
A bilayer seed structure having a non-magnetic metal seed layer with a FCC structure adjacent to a pinning layer in a spin valve sensor is described in U.S. Pat. No. 6,208,492. The bilayer seed structure increases the MR ratio but the effect on EM performance is not mentioned.
In U.S. Pat. No. 6,903,904, an AP2 layer is modified to form a multilayer structure by the insertion of at least one electron spin depolarizing layer such as Ta, Ti, Zr, or NiFeCr with an unspecified Fe content that minimizes the negative contribution from the AP2 layer to the GMR effect and thereby increases the MR ratio.
U.S. Pat. No. 6,818,331 discloses a FeTa layer as a soft magnetic undercoat that is formed on a substrate and below an orientation regulating layer. The FeTa layer increases the magnetic flux component from a magnetic head in a direction perpendicular to the substrate and improves the magnetic characteristics of the device.